This investigation is a family study on the effect of heterozygosity for an inactivating mutation of the estrogen receptor gene on bone density and other parameters of estrogen action. Excluding one nuclear family, the lumbar spine bone densities suggest that carrier status is associated with decreased bone mineral density compared to wild type; the mean spine BMD z scores were -0.9 for heterozygotes vs. -0.1 in wild type individuals (p=0.02).